This paper describes an evaluation of possible changes in sexual behavior in adolescents who participated in a school-based sex education program in selected public schools in four municipalities in the state of Minas Gerais, Brazil. The program is inserted within the context of reproductive rights, deals with risks involved in unsafe sexual practices and focuses on the positive aspects of sexuality. A quasi-experimental design with pre and post-tests and a non-equivalent control group was used to evaluate the intervention. A total of 4,795 questionnaires were included in this analysis. The program succeeded in more than doubling consistent condom use with casual partners and in increasing the use of modern contraceptives during last intercourse by 68%. The intervention had no effect on age at first intercourse or on adolescents' engagement in sexual activities. The sex education program was effective in generating positive changes in the sexual behavior of adolescents, while not stimulating participation in sexual activities.